Before an answer can be provided, however, we must admit that baptism can be a divisive subject for Christians, especially when it comes to the proper recipients of the sacrament.  Let me offer a few reasons why this might be the case:
 
1.  Misunderstandings
Misunderstandings promote division and there are many misunderstandings about infant baptism. It is important for us to point out that just because a church practices infant baptism does not mean that it refuses to baptize adults as well.  In my initial pastoral experience as a minister in Scotland I discovered that this was a very common misunderstanding among Credobaptists (those who exclusively practice believer’s baptism.)  The assumption seemed to be that Paedobaptists (those who practice infant baptism) deny the sacrament to adults.  This assumption is false.  All Paedobaptists (to my knowledge at least) believe that adult converts from non-Christian backgrounds are to be baptized according to the clear example set forth in the New Testament.  
 
2.  Cultural Norms
We have to acknowledge a particular cultural difficulty in addressing the issue of Paedobaptism.  We are Americans and the fact is that most American evangelicals do not practice infant baptism.  Instead they prefer to baptize only those who have verbally professed faith in Christ.  But it is important for American evangelicals to understand that though their baptismal practice is the majority position in America evangelical churches it is the minority position throughout the rest of the world and throughout the history of the Church.
 
3.  Textual Difficulties
It is proper to acknowledge the textual challenges that exist for anyone attempting to define and defend their baptismal practice regardless of what that practice might be.  For we Paedobaptists this means we must honestly admit that there is no passage in the New Testament explicitly commanding us to baptize infants.  However, those who oppose Paedobaptism must also acknowledge that there is no passage explicitly forbidding the baptism of infants, let alone affirming the baptism of only those capable of verbally professing their faith.
 
With these preliminary considerations out of the way I think that we can now make a brief case for infant baptism by offering a threefold answer to the question that entitles this article.

Answer #1:  Because we bear this striking fact in mind – baptism is a sacrament and sacraments are not a new covenant novelty.  Indeed, the new covenant sacraments (baptism and communion) are rooted in the sacraments of the old covenant (circumcision and the Passover) and are to be understood according to their institution.  Regarding the latter the Old Testament offers no standard chronology for faith and circumcision.   For example Abraham was circumcised after he verbally professed faith (Gen. 17:22–27), but Isaac, Abraham’s son, was circumcised before his confession (21:4).  Yes, faith in the Lord was necessary in both of their cases.  Faith in the Lord was needed to appropriate all the benefits and blessings that circumcision promised, but the administration of circumcision as the sign and seal of those promised blessings was not tied to the timing of these men’s faith.  Because the New Testament links circumcision to baptism (Col. 2:8–15), Paedobaptists conclude that baptism, like circumcision, does not need to be tied to the timing of one’s profession of faith.

 
Answer #2:  We believe that the new covenant was foreshadowed by the old covenant and many of the principles of the old covenant still apply to the new.  One of these principles embraced by Paedobaptists concerns the fact that in the old covenant era God’s promises were given not just to adults but also to their children.  This reality was powerfully depicted in the Old Testament sacrament of circumcision.  Paedobaptists argue that it is hard to imagine that the far greater new covenant promises that we enjoy today in Christ should not also be given to the infant children of believing adults.  Such a diminishing of promised blessing seems even harder to imagine when one reads the Apostle Peter telling his audience on the day of Pentecost that the new covenant promises, like the old, are gifts for both believers and their children. (Acts 2:38–39).  Also there’s the odd fact that over a quarter of all baptisms recorded in the New Testament are of entire households.  Though it is admittedly an argument from silence can we reasonably assume that not one of these entire households had infants and/or small children in them?  Paedobaptists believe that such an assumption would be unreasonable.

​ 
 
Answer #3:  We Paedobaptists take seriously the New Testament’s clear teaching that the children of believing parents are set apart unto God (1 Cor. 7:12–14).   Under the old covenant it was circumcision that visibly set the children of believers apart from the rest of the world.  Paedobaptists believe that God, in the new covenant, still sets the children of believing parents apart from the rest of the world, marking them out as part of the visible community of God’s people through the waters of baptism.  Now it may be that my Credobaptists friends would object to this by pointing out that baptizing infants who lack personal faith in Christ is a worthless enterprise, but Paedobaptists would argue otherwise.  On the one hand it is very true that ultimately baptism without personal faith avails nothing. But on the other hand, the baptism of infants is far from worthless.  By the grace of God baptism marks out the children of believing parents as being really and truly part of the visible church, as heirs to all of the promises of God in Christ, and therefore responsible to embrace those promises by laying hold of Christ by faith as they grow in years and understanding.
 
In conclusion, “why do you baptize infants?”  Here’s why.  We believe that Paedobaptism is a beautiful and biblically defensible practice.  We also believe that the children of Christian parents have a particular relationship to the Lord that the children of unbelievers do not possess (1 Corinthians 7:12–14).  Furthermore, we believe that the children of believing parents have been sovereignly brought into the visible church by virtue of their natural birth and that unlike other children they will grow-up surrounded by the means of grace and fed upon the Word of God. 
 
To state it as simply as possible we baptize infants because we believe that they are part of God’s Kingdom too.   ​

The New Testament book of Hebrews is a wonderful testimony to the person and work of the Lord Jesus Christ.  For 12 chapters the author points his readers again and again to Christ as the only savior for sinners and as the personal fulfillment of redemptive history itself.  As he concludes his letter in the 13th chapter the author states an important fact about the Lord Jesus followed by a crucial piece of instruction for Christians: 

If Jesus Christ is the same yesterday and today and forever then it only stands to reason that the doctrine of salvation - which is grounded in the person and work of the Lord Jesus - is likewise unchanging.  This is why the author of Hebrews says to his readers "do not be led away by diverse and strange teachings."  As Christians it is of vital importance that we uphold and maintain an orthodox understanding of what it means to be saved.  Like the Savior Himself, the biblical doctrine of salvation has not changed and will not change...  so let's make sure that we know this doctrine in all of its parts to the glory of God. 

One tool to help us understand the doctrine of salvation is called the "Ordo Salutis," (Latin for Order of Salvation).  The Ordo Salutis lays out the logical sequence of salvation from a biblical perspective.  It includes important biblical words that all Christians should be able to define - words like:

Election
Effectual Calling
Regeneration
Conversion
Justification
Adoption
Sanctification
Perseverance
Glorification

How many of these words do you know?

To help you learn these words or to refresh your understanding of them Tim Challies has produced a wonderful infographic illustrating the Ordo Salutis - you'll find it below.

Click here for a higher-quality printable version of the infographic.